There has been an electric power steering apparatus (EPS) as an apparatus that is provided a motor in a driving section, and the electric power steering apparatus gives a steering assist force (assist force) to a steering mechanism of the vehicle through a rotational force of the motor, and gives a motor driving force controlled with an electric power supplied from an inverter as a steering assist force to a steering shaft or a rack shaft by using a transmission mechanism of gears and so on. In order to accurately generate the assist torque of a steering assist force, such a conventional electric power steering apparatus performs the feedback control of a motor current. The feedback control adjusts the voltage supplied to the motor so that the difference between a current command value and a detected motor current value becomes small, and the adjustment of a motor applying voltage is generally performed by the adjustment of a duty ratio of a PWM (Pulse Width Modulation) control, and a brushless motor that is superior in durability and serviceability and is low noise, is commonly used as the motor.
To describe the general construction of the electric power steering apparatus is shown in FIG. 1, a handle 1 of a column shaft (steering shaft, handle shaft) 2 is connected to steered wheels 8L, 8R through reduction gears 3 in a reducing section, universal joints 4a and 4b, a pinion rack mechanism 5, tie rods 6a and 6b, further via hub units 7a and a 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Ts of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angel 6, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of the assist (steering assist) command based on the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12, and controls a current to the motor for EPS based on a voltage control command value Vref by performing a compensation and so on with respect to the current command value.
Further, the steering angle sensor 14 is not indispensable and may not be provided, and it is possible to obtain the steering angle from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from CAN 40) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 that is the target value of the current supplied to the motor 20 based on the steering torque Is and the vehicle speed Vs and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A as a current command value Iref2. A current command value Irefm that is limited the maximum current, is inputted into a subtracting section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im that is fed back, is calculated. The deviation I is inputted into a PI-control section 35 to improve the characteristic of the steering operation. The voltage control command value Vref that characteristic improvement is performed in the PI-control section 35, is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a drive section. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. In general, the inverter 37 uses EFTs as driving elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensating section 34 is added to the adding section 32A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensating section 34 adds a self-aligning torque (SAT) 34-3 and an inertia 34-2 to an adding section 34-4, further adds the result of addition performed at the adding section 34-4 and a convergence 34-1 in an adding section 34-5, and then outputs the result of addition performed at the adding section 34-5 as the compensation signal CM.
In such the electric power steering apparatus, cases being used a motor having multi-system windings of a constitution to continue a motor operation even if a failure (including an abnormality) of the motor occurs, are increasing. For examples, with respect to a motor having 2-system windings, coils of a stator are separated to 2-systems (U1-W1 phases, first system and U2-W2 phases, second system), and it is possible to rotate a rotor of the other 1-system even if 1-system is failed and to continue the assist control.
In the motor control unit provided with such the motor and the electric power steering apparatus, for an example, a solution means in a case that an open-circuit failure of the motor windings or the inverter occurs, is disclosed in Japanese Published Unexamined Patent Application No. 2013-236486 A (Patent Document 1). That is, the apparatus disclosed Patent Document 1 relates to the motor control unit to control a current supplied from a power source and an applied voltage for the motor having winding group of the plural systems that: stops to supply a current to a winding that a failure occurred by a control of a switching element within the inverter at a failure side when a failure judging means judges an open-system failure, and continues a current-supplying to a normal winding that a failure does not occurs.